Air separation

Abstract

In order to provide `ultra high purity` nitrogen having diminished concentrations of light and heavy impurities in comparison with nitrogen produced by conventional cryogenic air separation, the nitrogen product from a conventional cryogenic air separation column is introduced into the bottom of a liquid-vapor contract column 2 fitted with a condenser 8 to provide reflux. A liquid nitrogen stream having a reduced concentration of heavy impurities is withdrawn from the column 2 through an outlet 22 situated at a level a few trays below the top tray in the column 2. The liquid nitrogen is then subjected to two stages of flash separation. In the first stage the liquid is passed through valve 24 into a phase separator 26. In the second stage, the resulting liquid from the first stage, having a reduced concentration of light impurities, is passed through valve 32 into a phase separator 34. Liquid nitrogen product is withdrawn from the phase separator 34 through outlet 38.

We claim: 
     
       1. A method of purifying nitrogen containing light impurities and heavy impurities comprising: introducing under pressure a stream of the nitrogen into a liquid-vapor contact column so that an ascending flow of nitrogen gas is produced within the liquid-vapor contact column; extracting the nitrogen gas from the top of the liquid-vapor contact column and condensing it to form a condensate; introducing the condensate into the top of the liquid-vapor contact column to produce a descending flow of liquid nitrogen; absorbing the heavy impurities into the descending flow of liquid nitrogen and stripping the light impurities therefrom into the ascending flow of nitrogen gas so that the descending flow of liquid nitrogen becomes progressively richer in the heavy impurities and leaner in the light impurities as it descends in the liquid-vapor contact column and the ascending flow of nitrogen gas becomes progressively richer in the light impurities and leaner in heavy impurities as it ascends in the liquid-vapor contact column; withdrawing from the liquid-vapor contact column a liquid nitrogen stream having a reduced concentration of heavy impurities and subjecting the liquid nitrogen stream to at least one stage of flash separation to produce a liquid nitrogen product having a heavy impurity concentration less than that of the nitrogen to be purified and a reduced concentration of the light impurities. 
     
     
       2. The method as claimed in claim 1, in which the liquid nitrogen stream is subjected to two of the at least one stage of flash separation. 
     
     
       3. The method as claimed in claim 1, in which the second nitrogen stream is subjected to three of the at least one stages of flash separation. 
     
     
       4. The method as claimed in claim 1, in which refrigeration for the condensation is provided by liquid oxygen. 
     
     
       5. The method as claimed in claim 1, in which the feed nitrogen stream is taken from the higher pressure column of a double column for separating air into oxygen and nitrogen. 
     
     
       6. The method as claimed in claim 5, in which the feed nitrogen stream is taken into the vapour state. 
     
     
       7. The method as claimed in claim 5, in which the feed nitrogen stream is taken in the liquid state and is reboiled upstream of where it is introduced into the liquid-vapor contact column. 
     
     
       8. The method as claimed in claim 1, in which a purified nitrogen product containing less than 0.1 volumes per million of impurities is produced. 
     
     
       9. An apparatus for purifying nitrogen containing light and heavy impurities and obtained from a feed nitrogen stream under pressure, said apparatus comprising: a liquid-vapor contact column having a bottom inlet for the feed nitrogen stream under pressure to produce an ascending flow of nitrogen gas within the liquid-vapor contact column; means for condensing the ascending flow of the nitrogen gas and for creating in the column a descending flow of liquid nitrogen, whereby the column is operable to absorb the heavy impurities into the descending flow of liquid nitrogen and to strip the light impurities therefrom into the ascending flow of nitrogen gas so that the descending flow of liquid nitrogen becomes progressively richer in the heavy impurities and leaner in the light impurities as it descends in the liquid-vapor contact column and the ascending flow of nitrogen gas becomes progressively richer in the light impurities and leaner in heavy impurities as it ascends in the liquid-vapor contact column; the liquid-vapor contact column also having an outlet for extracting a liquid nitrogen stream, the outlet located at a level of the column at which the liquid nitrogen has a reduced concentration of the heavy impurities; and means including at least one stage of flash separation for separating the light impurities from the liquid nitrogen stream and thereby producing a product stream having a heavy impurity concentration less than the nitrogen to be purified and a reduced concentration of the light impurities. 
     
     
       10. The apparatus as claimed in claim 9, in which there are two of the at least one stage of flash separation. 
     
     
       11. The apparatus as claimed in claim 9, in which there are three of the at least one stage of flash separation. 
     
     
       12. The apparatus as claimed in claim 9, in which the means for condensing the ascending flow of nitrogen gas and for providing the descending flow of liquid nitrogen is a condenser having as inlet for vapour in communication with the top of the column and an outlet for condensate in communication with the top of the column. 
     
     
       13. The apparatus as claimed in claim 12, in which the passages in the condenser in which in operation the nitrogen vapour is condensed communicate with an outlet for uncondensed vapour, whereby a bleed of uncondensed vapour is able to be discharged from the condenser. 
     
     
       14. The apparatus as claimed in claim 9, wherein the source of nitrogen is the higher pressure column of a double distillation column for separating air into oxygen and nitrogen. 
     
     
       15. The apparatus as claimed in claim 14, additionally including a reboiler for reboiling liquid nitrogen feed upstream of the said liquid-vapor contact column.